1. Field of the Invention
The present invention relates to a dual peak wavelength tube, an illuminator for inspection, an inspecting apparatus and an inspecting method. More particularly, it relates to an inspecting apparatus and an inspecting method suitable for visually inspecting the presence or absence of deformations on a surface of an object, such as a wafer, caused by local defocusing by irradiating the finely patterned surface of the object/wafer formed by the photolithography method with light, and a dual peak wavelength tube used for the method.
2. Description of Related Art
In a photolithographic process for producing a semiconductor device, various factors such as line width and superposition have to be controlled. In addition, other factors such as uniformity of resist thickness also have to be controlled to prevent defocusing problems, and the like. Visual inspections for defocusing, size measurement, and overlay measurement have been conducted as in-line inspections for wafers after a resist pattern is formed by each photomask. Among these inspections, it is generally enough for size measurement and overlay measurement to make sampling tests so as to reduce the frequency of the measurement, as far as the photolithographic process is stable.
Even though a visual inspection for defocusing is a visual sensory test, this inspection is usually performed on all lots. This is because the deformation of the resist pattern, which cannot be microscopically detected in the size measurement and the overlay measurement, can be detected in a visual inspection for defocusing. Defocusing herein means a phenomenon in which the deformation of the resist pattern is caused by offset of the focus when the resist pattern is exposed to light with a stepper.
There are various causes of defocusing, such as a local protrusion on the surface caused by dirt attached to the back side of the substrate, such as a wafer, defects such as scratches on the surface of the substrate itself, and distortion of the substrate, or the difference in the film thickness of a resist coated on the surface of the substrate or displacement of the stage, or the like, which will cause deformation in the resist pattern. The defocusing may cause abnormalities such as variations in the line width of a semiconductor layer etched by the resist pattern. Accordingly, for example, in the case of gate formation of an FET (Field effect transistor), the channel length becomes shorter as the gate length becomes shorter due to defocusing, which results in nonconforming operation speed of the transistor.
If deformations of resist patterns are detected at an early stage, it is possible to reuse the substrate by stripping the resist from the substrate. On the contrary, if this visual inspection for defocusing is not very effective, late abnormality detection of the apparatus will result in unrenewable yielding loss over several lots.
There was a problem that detection of nonconformity caused by defocusing was difficult, although the visual inspection for defocusing played a very important role. More particularly, this visual inspection for defocusing is an inspecting method using a phenomenon that micron-order lines of a resist pattern (finely patterned surface) function as a diffraction grating. When lights are applied to the lines, normal parts and deformed parts in the resist pattern diffract the lights in different manners, so that the color or brightness of diffracted lights appears differently. It was very hard to discern the difference because of a slight difference of the diffracted lights in color or brightness. Thus, visual inspections have required so far considerable time and experience.
Due to these circumstances, there have been proposals for various kinds of automatic inspecting apparatuses so far. In such inspecting apparatuses, however, a conventional halogen lamp was used, which was unable to clearly discern the lights diffracted from normal portions from the lights diffracted from abnormal portions. Consequently, even if the diffracted lights are separated to take out lights within a predetermined wavelength range, and the lights are electrically detected by being converted into electrical signals using an optoelectric transducer, the detection accuracy is not good because of difficulty in detecting slight differences.
One of the inventors of the present invention previously filed the cross-referenced application, Ser. No. 09/698423, entitled, xe2x80x9cDefocus Visual Inspection Toolxe2x80x9d, by inventors M. Uda et al. Lights illuminated by an illuminator for macro inspection according to the invention in the application comprise lights in two different colors having a sharp color contrast effect. With the use of the illuminator, lights reflected and diffracted from a portion where an abnormal resist pattern is included, and the lights reflected and diffracted from a portion where a normal resist pattern is included appear in two colors having a sharp color contrast effect. The visual inspection for reflected and diffracted lights, therefore, has made the detection of abnormal portions much easier. On the contrary, there was a fear of overlooking the abnormal portions because colors having no relation with color contrast effect might appear depending upon the direction of viewing the reflected and diffracted lights.
It is therefore an object of the present invention to provide methods and apparatus useful for inspection and other lighting applications. The methods use and the apparatus include, an illuminator, which is capable of visually detecting the presence or absence of defocusing and/or other problems on a surface of a semiconductor substrate, such as a wafer, on which resist patterns are finely formed, and widens particularly angle of visual field to improve visual inspection visibility.